We will use modules in all applications in this book. Modules (also called external modules and ES2015 modules) are a concept of separating code in multiple files. Every file is a module. Within these modules, you can use variables, functions, and classes (members) exported by other modules and you can make some members visible for other modules. To use other modules, you must import them, and to make members visible, you need to export them. The following example will show some basic usage:

// x.ts 
import { one, add, Lorem } from './y'; 
console.log(add(one, 2)); 
 
var lorem = new Lorem(); 
console.log(lorem.name); 
 
// y.ts 
export var one = 1; 
export function add(a: number, b: number) { 
  return a + b; 
} 
export class Lorem { 
  name = "ipsum"; 
} 

You can export declarations by prefixing them with the export keyword or by prefixing them with export default. A default export should be imported...